Lots of stuf can cause it including sediment or a blocked supply line
A leak in the supply pipe.
check for leaks
With no air in the system anywhere, there would be nothing to hold pressure, so the slightest leak would cause the system to go to zero.
Stopped at all faucets? Didn't pay the water bill. If in cold climate, the line from the outside to probably the water heater has probably frozen.
Possibly a pressure reducing valve installed after piping for first faucet, which would cause situation describing.
Air in the water lines to the upstairs. Not enough water pressure.
Some causes for a sudden water pressure drop: 1) A big leak on the main supply line. 2) one or more faucets or shower heads opened on the same water line. 3) A utility water pump failure.
Hot water tank needs bleed of air,
Lime buildup in your water lines. No, it wont hurt you.
old galvanized piping
I would purchase a Watt water pressure guage from your local home improvement or hardware store and check your water pressure at your faucet. If water pressure is greater than 40-45psi your regulator needs to be turned down where the water comes into the house.
It will not cause low water pressure. It will cause a loss of heated water because the heater will not be able to recover fast enough. It has nothing to do with pressure.